To increase the wear resistance of a ferrometallic part and thereby increase the useful life of such part it is well known in the art to harden the surface of the part, especially in selected regions, by increasing the carbon content of such surface along with a heat treatment of the part. This technique is known as case hardening. The prior art methods of case hardening ferrometallic parts typically involves carburizing the surface of the part by heating it in contact with a solid carbon containing material or in a gaseous atmosphere containing a source of carbon. This step produces a carburized part (i.e. a part whose carbon content at the surface region has been increased significantly over its original carbon content and that of the carbon content of the core region of the part). The part is then heated at a hardening temperature for some period of time. After being heated to the hardening temperature the part is then quenched (i.e. cooled) by immersion in oil or water or cooled gradually by exposure to ambient air. This process produces chemical, microstructural, mechanical and physical changes in the surface region of the part.
Increasing the carbon content in the surface region of a ferrometallic part along with the heat treatment, in the case hardening process, produces a hard wear resistant surface which can be more brittle than the softer, tougher, lower carbon content regions of the part. Such a hard surface is often desirable in obtaining a part having a long useful life. It is often desirable in the art to produce ferrometallic parts having hard, wear resistant surface regions adjacent to softer, tougher regions. For example, it may be desirable to produce a metal gear having hard wear resistant teeth surfaces and a tough, non-brittle hub. Selective case hardening is a means for producing such a part. In the prior art case hardening methods such a gear may be obtained by coating the portion of the gear that is not to be carburized (i.e the hub) with a carbon impervious material (e.g. copper plating or fire clay) to prevent carbon from penetrating into the surface at that portion of the gear during the carburizing step The remaining uncoated portion of the gear (i.e the teeth) is exposed to a carbon source (e.g. a gaseous atmosphere having a source of carbon) during the carburizing and heat treating steps and thus picks up carbon to create a hard, wear resistant surface. The amount of carbon picked up by the uncoated portion of the gear is essentially established in the carburizing step and is maintained in the heat treating step by a gaseous atmosphere whose carbon content is essentially the same as the carbon content of the carburized portion of the gear. Thus in the heat treating step, with such a carbon containing atmosphere, the concentration of carbon in the carburized portion of the gear is kept essentially constant. After the heat treating step and quenching the coating is removed from the non-carburized portion of the gear. The use of copper plating has temperature limitations in the carburizing and heat treating steps to prevent burning off of the layer of copper. Cyanide compounds are often used in connection with the plating and removal of the copper layer. Such compounds are known to be toxic. Fire clay and other known art methods of protecting portion of a ferrometallic workpiece from carburization and hardening present other individual application and removal problems. In general the coating and other protective steps are time consuming and costly.
In the carburization step of the case hardening process the ferrometallic part is heated while being exposed to carbon containing materials in a solid or gaseous state. The present state of the art principally employs a gaseous atmosphere containing a source of carbon in the carburizing and heat treating steps. During the carburizing step carbon is absorbed into and penetrates the exposed surface regions of the part. The amount of carbon absorbed and the depth of penetration of the carbon into the part are dependent upon such factors as part configuration and dimensions, temperature, time, composition of the metal (e.g. alloying agents) and the material acting as the source of carbon. Generally the penetration of carbon into the part is kept to a limit of one tenth of an inch. This depth of penetration is of course established by factors such as part thickness, degree of hardness and intended use of the part. Alloying agents in the ferrometallic part, such as chromium, nickel, manganese, silicon, phosphorus and sulfur are well known to have an effect on the amount of carbon taken up and the rate and depth of penetration of carbon into the surface during the carburizing step and the structure of the hardened metal after the heat treating step. Chromium tends to promote absorption of carbon and can lead to a fine grained structure in the hardened metal.
The heat treating of the case hardening process of the art involves heating the carburized part to a particular temperature or temperature range, holding the part at that temperature for a specified time and cooling the part rapidly or gradually. Heating the part is carried out in contact with a source of carbon, usually a gaseous atmosphere having a carbon source and a high carbon content that minimizes the loss or gain of carbon in the carburized region of the part. Rapid cooling of the part is accomplished by immersion in oil or water. Slow cooling of the part may be done by exposure to air under ambient thermal conditions.